The 24-hour clock is the convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today, and is used by international standard ISO 8601.
A limited number of countries, particularly English-speaking nations, use the 12-hour clock as a standard, or a mixture of the 24- and 12-hour time systems. In countries where the 12-hour clock is still dominant, some professions prefer to use the 24-hour clock. For example, in the practice of medicine the 24-hour clock is generally used in documentation of care as it prevents any ambiguity as to when events occurred in a patient's medical history. In the United States and a handful of other countries, it is popularly referred to as military time.[
WWV is the call sign of the United States National Institute of Standards and Technology's (NIST) HF ("shortwave") radio station located near Fort Collins, Colorado. WWV continuously transmits official U.S. Government frequency and time signals on 2.5, 5, 10, 15, and 20 MHz.
These carrier frequencies and time signals are controlled by local atomic clocks traceable to NIST's primary standard in Boulder, Colorado by GPS common view observations and other time transfer methods. NIST also operates the very similar radio station WWVH in Kauai, Hawaii. WWV shares its site near Fort Collins with radio station WWVB which transmits carrier and time code (no voice) on 60 kHz in the LF ("longwave") band.
Both WWV and WWVH announce the Coordinated Universal Time each minute, and make other recorded announcements of general interest on an hourly schedule, including the GPS satellite constellation status and severe oceanic weather warnings. Since they share frequencies, WWV uses a male voice to distinguish itself from WWVH, which uses a female voice. WWV time signals can also be accessed by telephone.